title: Highly methyl-branched hydrocarbon surfactant as a CO₂-philic solubilizer for water/supercritical CO₂ microemulsion.
abstract: To develop an efficient and fluorine-free solubilizer for a water/supercritical CO₂ microemulsion (W/CO₂ μE), in this study, a highly methyl-branched alkyl, isostearyl group was focused on as a CO₂-philic tail, and the custom-made isostearyl surfactant, sodium 2-(4,4-dimethylpentan-2-yl)-5,7,7-trimethyloctyl sulfate (SIS1) was synthesized. The surface tension (γ) of an aqueous SIS1 solution was measured at ambient pressure as a function of surfactant concentration, and it was found to be 25 mN/m at concentrations of > 1.5 mM. A low γ value can generally be reached only by a fluorocarbon surfactant, which implies that SIS1 has an excellent solubilizing power for the W/CO₂ μE, similar to some fluorocarbon surfactants reported previously. Visual observations of the SIS1/W/CO₂ mixtures revealed the formation of transparent single phases without separated water, identified as W/CO₂ μE. The μE was well-stabilized at pressures > 210 bar and temperatures > 55 °C. At 75 °C and 370 bar, SIS1 was found by spectral measurements using a water-soluble UV-light absorber to solubilize water contents up to a maximum water-to-surfactant molar ratio (W0) = 50. The achievement of W0 = 50 in a W/CO₂ μE system has not been reported previously in similar hydrocarbon surfactant/W/CO₂ systems, and this demonstrates that a highly methyl-branched alkyl group can act as a good CO₂-philic group for a W/CO₂ -type surfactant.
authors.affiliation: Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki UniversityHirosaki, 036-8561, Japan. sagisaka@cc.hirosaki-u.ac.jp
authors.forename: Masanobu
authors.initials: M
authors.lastname: Sagisaka
authors.forename: Kotaro
authors.initials: K
authors.lastname: Kudo
authors.forename: Shota
authors.initials: S
authors.lastname: Nagoya
authors.forename: Atsushi
authors.initials: A
authors.lastname: Yoshizawa
mesh_terms: D002245:Carbon Dioxide / Q000737:chemistry*; D004655:Emulsions; D013229:Stearic Acids / Q000737:chemistry; D013500:Surface Tension; D013501:Surface-Active Agents / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D004655:Emulsions; D013229:Stearic Acids; D013501:Surface-Active Agents; D014867:Water; D002245:Carbon Dioxide; C040995:isostearic acid